The present invention relates to a laser spectroscopy system, particularly to a spectroscopy system using a tunable laser diode as the optical source for analyzing a very small amount of ingredients included in a gas through infrared spectroscopy.
Conventionally, an infrared spectroscopy system is widely used as an analyzer for analyzing ingredients in the gaseous sample, for example analyzing impurities included in a gas sample. The conventional infrared spectroscopy is the technique to measure an absorption spectrum by transmitting an infrared beam into the sample gas and to analyze this absorption spectrum, so that it is possible to identify the molecules (impurities) to be measured in the sample gas from the wavelength of the absorbed beam in the absorption spectrum and to determine the amount of the molecules from the amount of absorbed beam in the absorption spectrum. Particularly, since it is known that the conventional spectroscopy which uses a near-infrared laser diode as the optical source has high sensitivity and precision, the conventional spectroscopy is used for determining the small amount of water molecules in the semiconductor material gases manufactured or used in the field of semiconductor industry and the related materials industry, or diagnosing diseases by analyzing the stable isotopes in the patients"" exhalation.
FIG. 3 is a diagram which shows a general configuration of a conventional spectroscopy system using a laser diode as the optical source. The spectroscopy system shown in FIG. 3 includes an optical system which has a tunable laser diode source 10 for generating a laser beam for measurement, a sample cell 11 where a sample gas is introduced, the first photo detector 12 for measuring an intensity of a laser beam transmitted through the sample cell 11, two beam splitters 13 and 14 for splitting a portion of the laser beam from the laser source 10, the second photo detector 15 for measuring an intensity of a laser beam splitted (reflected) by the first beam splitter 13, a reference cell 16 where an object to be measured is introduced under depressurized condition, and the third photo detector 17 for measuring an intensity of a laser beam splitted (reflected) by the second beam splitter 14 and transmitted through the reference cell 16. Generally, this optical system is contained in a purge box 18. Further, the laser source 10 has driving means 10a and 10b for controlling driving current and operating temperature. The photo detectors 12, 15 and 17 respectively have pre-amplifiers 20 for converting the detected amount of laser beams into electrical signals, amplifying the signals and outputting them to lock-in amplifier 19.
According to the conventional laser spectroscopy system, the object gas to be measured is introduced in the reference cell 16 at a predetermined pressure, for example about 100 Torr and the sample gas flows through the sample cell 11 at a predetermined pressure, for example about 100 Torr. Under this condition, a laser beam of a predetermined wavelength is generated by the laser source 10 via the driving means 10a and 10b under the control of the control means 21, such as a personal computer. The amount of detected laser beams by the respective photo detectors 12, 15 and 17 are inputted to the control means 21 through the lock-in amplifier 19, and the amount of ingredients to be measured in the sample gas is acquired by calculations. The laser beam from the laser source 10 is irradiated as dispersion is removed by adjusting the diameter of the beam while passing the lens 22 or slit, pinhole or the like.
FIG. 4 is a diagram of an example of second derivative absorption spectra for measuring concentration of water molecules in hydrogen chloride by using the conventional laser spectroscopy system. The uppermost second derivative absorption spectrum X shows an absorption intensity of the laser beam detected by the first photo detector 12, wherein the laser beam is transmitted through the beam splitters 13 and 14 and the sample cell 11. The middle second derivative absorption spectrum Y shows an absorption intensity of the laser beam reflected by the beam splitter 13 and detected by the second photo detector 15. The lowermost second derivative absorption spectrum Z is acquired by subtracting the absorption intensity detected by the second photo detector 15 from the absorption intensity detected by the first photo detector 12, and is an absorption intensity of the water molecules in the sample gas flowing through the sample cell 11. According to what is described above, it is possible to cancel the absorption intensity of the beam other than that in the sample cell 11 line and to acquire only the absorption intensity of the water molecules in the sample gas in the sample cell 11 by subtracting the absorption intensity detected by the second photo detector 15 of the cancel line from the absorption intensity detected by the first photo detector 12 of the so called sample line. Therefore, it is possible to calculate the concentration of the water molecules in the hydrogen chloride by reading values of peak and valleys of the second derivative absorption spectrum Z.
In the real measurement, however, since it is rare to get such a clear second derivative absorption spectrum as shown in FIG. 4 and there is an undulation called xe2x80x9cfringe noisexe2x80x9d in the ordinary second derivative absorption spectrum, it is very difficult to measure a very small amount of ingredient with high precision. For example, FIG. 5 is a diagram of an example of second derivative absorption spectra of a refined and dehydrated hydrogen chloride flowing through the sample cell 11. As before, the lowermost second derivative absorption spectrum Z is acquired by subtracting the middle second derivative absorption spectrum Y from the uppermost second derivative absorption spectrum X. As shown in FIG. 5, though there is no water molecule in the sample gas, there is a large undulation by fringe noise in the second derivative absorption spectrum Z, so that there is a peak at the wavelength of water molecule""s line.
This fringe noise is generated when the laser beam is transmitted or reflected through/by the inside wall and windows of the sample cell 11 and/or the beam splitters 13 and 14. When this fringe noise is generated, the measuring precision is deteriorated because a large distortion is generated in the valley area. For example, as shown in FIG. 6, if the fringe noise becomes larger, the peak P of water molecules, which originally would be represented as the upper spectrum of FIG. 6, is buried by the fringe noise Q, so that the measurement becomes difficult. Further, when other ingredient, such as carbon dioxide or hydrogen bromide in case of water molecule, of which the absorption wavelength is similar to that of the water molecule, exists, the peak R of the hydrogen bromide is located near the peak P of the water molecule, and it becomes difficult to distinguish the peaks and to perform precise measurement. These above described problems become much more serious particularly when a very small amount of impurities in a highly purified gas is analyzed.
Therefore, when analyzing water molecules, a 100% of water moisture is installed in the reference cell 16 with a prescribed pressure and the absorption wavelength of water molecule is identified by detecting the laser beam transmitted through the reference cell 16 by the third photo detector 17. In other words, even when the peak of the second derivative absorption spectrum Z is as small as the fringe noise, it is possible to clearly grasp the peak of absorption spectrum of the laser beam transmitted through the sample cell 11 by referencing the peak of the laser beam transmitted through the reference cell 16. As a result, it is possible to measure the amount of the water molecules with high precision. Further, by providing the reference cell 16 and the third photo detector 17, called reference line, even when the other ingredients of which the absorption wavelengths are similar exist, it is possible to clearly measure the only amount of water molecules.
However, because there is provided the reference cell 16 and the laser beam is splitted by the second beam splitter 14 on the beam path, the power of the laser source 10 should be sufficiently large and this causes not only cost up but also larger fringe noise.
Further, as shown in FIG. 8, a focusing lens 23 is provided at the rear of the laser source 10 in order to converge the rear dispersion of the laser source 10, and the converged laser beam is irradiated to the reference cell 16 and detected by the third photo detector 17. In this case, however, since additional elements are provided on the axis of laser beam, the whole system becomes larger and more space is required.
In case the optical system is contained in a purge box 18, the volume of the purge box 18 should be increased due to installment of the reference cell 16. In order to change the atmosphere in the purge box 18, for example from the air to nitrogen atmosphere with water adjustment, the purging efficiency is decreased and the required time for purging is increased. Therefore, the system setup time becomes longer and the consumption of nitrogen gas is increased.
Because the reference cell 16 is provided, it is needed to add the beam splitter 14 and the focusing lens 23, and the number of required elements is drastically increased and the manufacturing cost is also increased.
Therefore, it is an object of the present invention to provide a laser spectroscopy system of simple construction and free of the influence of the fringe noise.
It is another object of the present invention to provide a laser spectroscopy system in which a reference cell is efficiently installed with minimum cost and space.
In accordance with an aspect of the present invention, there is disclosed a laser spectroscopy system including: a tunable laser diode source for generating a laser beam used for spectroscopic analysis; a sample cell where a sample gas is introduced; a first photo detector for measuring an intensity of a laser beam transmitted through the sample cell and having a beam receiving face; a beam splitter for splitting a portion of the laser beam from the laser source; and a second photo detector for measuring an intensity of a splitted laser beam from the beam splitter and having a beam receiving face, wherein the at least one of beam receiving faces is tilted to be at a predetermined angle from an axis of laser beam.
Further, the laser spectroscopy system according to the present invention further includes a reference cell, where an object to be measured is introduced, being positioned on a beam path of a laser beam reflected from the beam receiving face of the at least one of the photo detector of which the beam receiving face is tilted; and a third photo detector for measuring an intensity of a laser beam transmitted through the reference cell.